In an electrophotographic method, an image is generally formed through a step of developing an electrostatic latent image formed on a photoreceptor by attaching thereto a toner, a step of transferring the toner image onto a paper or a plastic film as a transfer material, and a step of fixing the transferred toner image to the transfer material by heating, etc. As a developer used for the electrophotographic method, there are a two-component developer comprising a toner and a carrier, a non-magnetic one-component developer comprising a toner only, and a magnetic one-component developer comprising a toner only. The two-component developer has a feature that the controllability is good, etc., because the carrier made of magnetic particles takes partial functions of charging the toner and transporting the toner, and has been widely used at present.
On the other hand, in a printer and a copying machine using an electrophotographic method, because of the progress of color copying and the improvement of the resolving power of the apparatus, the formation of a precise electrostatic latent image has been carried out. With the tendency, as a toner capable of faithfully developing the precise electrostatic latent image and forming an image of a higher image quality, a small particle size toner has been given attention recently. Particularly, in a full-color copying machine of developing a digital latent image with colored toners, and transferring and fixing the developed image, a high image quality to a certain extent has been attained by employing small particle toners of from 7 to 8 .mu.m, but in order to meet the requirement of further improvement of the high resolving power (the improvement of the fine-line reproducibility, the improvement of the gradation reproducibility, etc.), it becomes necessary to make the toner further small-sizing and also to make the particle size distribution of the toner an appropriate range.
However, when the particle size of toner is more reduced, a non-electrostatic attaching force typified by a van der Waals force is increased and the cohesive force of toner particles to each other is increased. Thereby, there is a tendency of reducing the powder fluidity of the toner and increasing the attaching force of tone to the carrier surface and the surface of photoreceptor, and as the result thereof, the developing property and the transferring property of the toner are reduced, the image density is lowered, and the cleaning property of the toner remaining on the surface of the photoreceptor is greatly lowered. Accordingly, it is the actual circumstances that the formation of the image having a high image quality by using a small-sized toner of 6 .mu.m or smaller has not yet attained.
Now, by using a small-sized toner, the image quality is improved by that the fine-line reproducibility and the gradation reproducibility are improved, and also the small-sized toner can contribute that improvement of the image quality in the point of reducing the toner weight of the imaged region. Hitherto, the toner layer of the imaged region becomes thick (particularly remarkable in color images) and in the case of fixing the image by a fixing roller, etc., a luster unevenness occurs in the imaged portions and the non-imaged portions, which substantially lowers the image quality. Thus, if the toner can be small-sized, the toner layer of the imaged region can be thinned and the occurrence of the luster unevenness can be restrained.
On the other hand, even when the toner weight of the imaged region is reduced by small-sizing the toner, of the colored density of the toner per unit weight thereof is same as that of a conventional toner, as the result thereof, the image density is lowered. As a method of preventing lowering of the image density, there is a method of increasing the concentration of the pigment contained in the toner. However, when the concentration of carbon black mainly used as the pigment of a black toner is increased, it gives an influence on the charging property of the toner. As the result thereof, it sometimes happens that a charging inferiority of the toner occurs and a charge is injected in the toner at development to form a fog on the image.
As a method of producing a black toner, there is a method of coloring a black color by mixing three kinds of pigments other than carbon black, such as, cyan, magenta, and yellow pigments. According to this method, because carbon black giving an influence on the charging property of a toner is not used, when the concentrations of the pigments in the toner are increased, the occurrence of the fogging phenomenon of image with the charging inferiority of the toner can be prevented. However, when these three kinds of the pigments are mixed to color a black color, there is a so-called color rendering wherein the black color is viewed as a different color according to the wavelength of the irradiating light and thus there is a problem that the viewing color is changed between under the sun light and under a light of a fluorescent lamp, etc., in room. Also, when the concentrations of the pigments in the toner are increased, it sometime happens that the properties of the toner such as the molten viscosity, etc., are changed, which causes lowering of the image quality in a fixing step.
Furthermore, in the toner of prior art, even when the addition amount of a coloring agent such as carbon black, etc., is increased, the graininess (rough viewing of the highlight portion) is deteriorated, whereby there is a problem that the image quality is rather lowered.